Operating System updates sometimes require reboots, and no one likes reboots. Reboots can be range from annoying for casual users to business-impacting for mission-critical business workloads. In spite of improvements to reduce their frequency, reboots will continue to be required for the foreseeable future. Please vote and comment if you want this topic covered in the Windows IT Pro Insider.

Definitely. Interested in how this changes with Windows 10 and the new Windows Update for Business.

Hi TH-,

Installing the serurity update will prompt to restart the machine. It is is reasonable.

The updates may update a DLL, .exe file, a device driver, registry entries that are read only when you start your computer. They couldn`t be updated if they are running.

Here is a link for reference, we can take the suggestions included in the link to reduce the chances to restart the machine.
Why you may be prompted to restart your computer after you install a security update on a Windows-based computer
https://support.microsoft.com/en-us/kb/887012

Best regards